Cold weather curing challenges for high-rise concrete slabs 3

[Sibroe]

Hi everyone,

I'm currently working on a high-rise concrete residential tower in Canada, and I'd like to hear your insights on flat slab curing in cold weather.

The structure is being built through the winter at higher floors (20+), and we've noticed that the slabs aren’t receiving any curing. Flying forms are being stripped 24–48 hours after the pour, and heating is applied from the floor below. That means there’s no curing on either face of the slab, and no insulating blankets on top, since the workers are already on that floor the next day.

The contractor claims that all high-rise projects in the area follow the same approach, as there’s no curing compound that works effectively in cold temperatures, meaning proper curing would require pausing work for a few days at each level.

We’re concerned about the potential impact of this on durability and deflections (2-way slabs, 215mm thick, 6m spans). While I know this lack of curing isn’t compliant with the code, nor ideal, is it really a significant issue? Is skipping curing really common and accepted practice in cold weather high-rise construction?

Side note : This is my first post here, but I’ve read hundreds of discussions on this forum over the years and have learned so much. Huge thanks to everyone who shares their knowledge! And a special shoutout to KootK! You’re a legend in my office!

 

[ANE91]

What are your cylinder breaks like?

Historically, the biggest issue I’m aware of is shoring/reshoring that was too expedient for the strength gain of the slabs. Towers have collapsed.

Is ACI 306.1 incorporated into your construction documents? If so, then they have to cure “properly.”

You could have finish issues if, say, you’ve spec’d for polish.

 

[Sibroe]

What are your cylinder breaks like?

Historically, the biggest issue I’m aware of is shoring/reshoring that was too expedient for the strength gain of the slabs. Towers have collapsed.

Is ACI 306.1 incorporated into your construction documents? If so, then they have to cure “properly.”

You could have finish issues if, say, you’ve spec’d for polish.

They typically use Lok-Tests for formwork removal and cylinder breaks later. Concrete strength has been over 75% of f’c at removal, around 95% at 7 days, and exceeding 100% at 28 days, so strength gain is not an issue.

As for shoring and reshoring, the general contractor is checking this and reports that the approved procedure is being followed (removing a single fly and reshoring immediately before stripping adjacent fly).

Surface finish is fine in this case.

Regarding ACI 306.1, we don’t reference it since this is a Canadian project, but I took a look at the guide. It doesn’t align with typical high-rise construction practices.

 

[ANE91]

Regarding ACI 306.1, we don’t reference it since this is a Canadian project, but I took a look at the guide. It doesn’t align with typical high-rise construction practices.

306 is the guide; 306.1 is the spec. Now you got me curious — which parts don't align? Hot mix water is common in the northeast US.

 

[Sibroe]

306 is the guide; 306.1 is the spec. Now you got me curious — which parts don't align? Hot mix water is common in the northeast US.

Both ACI 306 and 306.1 don’t address methods for ensuring proper curing while allowing work to continue on top of the slab during this period. I'm all for good concrete curing, but I also want to go beyond theory and understand the actual industry practices that balance durability with construction efficiency and tight scheduling.

 

[Enable]

I do shoring and re-shoring designs in the Ontario market for these kinds of projects. In general, curing methods are less than ideal in all seasons due to the rapid pace of construction. Somewhat ironically then, I have found that conventional curing protocols in the winter actually produce the best results due to moderate slab temperatures that reduce the drying surface potential. If your site-cast cylinder results are satisfactory, I would have no concerns. BTW the site-cast cylinders should be cast and held in the EXACT conditions of the slab (e.g. not down below in a cooler box but above the formwork surface) if you are using those to assess validity of the on-site curing procedures.

Most commonly the slab is heated from below with tarps enclosing the entire perimeter. It is so hot that the workers usually wear t-shirts even if it is -20 or so outside. This heat causes quite the thermal gradient across the slab such that even in moderate sub-zero ambient temperatures, the slab will be entirely above 0 and most likely above 5 degrees. Thus, the induced gradient plus heat of hydration allows the slab to cure while not freezing. That is the key, you cannot let the slab freeze prior to achieving 7mPa. If the ambient temperatures are so cold that this is a concern, then you'll absolutely require heating blankets and an accelerator in the mix.



Another issue is that interior slabs are usually not air entrained, and so it becomes more critical to ensure that snow is not allowed to melt and subsequently freeze on the surface (especially repeatedly). However, in terms of the actual cure itself, if you are using certain types of concrete then all CSA A23.1 requires (type 1 or 2) is that temperatures are controlled, not that they are wet cured. This is what contractors rely on generally in the winter (that they control temperatures and that the slab has reduced drying potential so that evaporation does not occur at a harmful rate - thus providing a form of moisture loss prevention). Here's a link to the RMCAO best practice guide.

 

[Sibroe]

Thanks for your detailed response Enable!

I realize my initial post wasn’t clear enough. My main concern isn’t about temperature control but rather moisture retention. Both CSA A23.1 and ACI 308 specify that curing isn’t just about temperature but also maintaining moisture conditions.

ACI 308 defines curing as "an action taken to maintain moisture and temperature conditions in a freshly placed cementitious material."

CSA A23.1 states that curing must "provide the temperature and moisture conditions for the period of time necessary for concrete to develop its strength, durability, and other properties."

To my understanding, a method of moisture retention is always required for all concrete, even under Type 1 and 2 curing in CSA A23.1. It just doesn’t necessarily have to be wet curing.

If there’s no moisture retention method, isn’t there a risk for concrete performance (even if tests show adequate strength)?

 

[ANE91]

If there’s no moisture retention method, isn’t there a risk for concrete performance (even if tests show adequate strength)?

Yes, so it’s up to you to figure out whether this is consequential enough to the building that your client expects to use. Best to make them aware of what overly dry slabs could mean for them. Besides strength and finishing, you’ve identified possible deflection issues associated with cracking and lower early-age stiffness and possible durability issues (e.g., freeze-thaw cycles, potentially porous/less-dense concrete, etc.).

 

[Enable]

I understand your concern, and it’s absolutely valid! But it’s also why I was trying to frame things in terms of reducing drying potential. You do not need to employ external moisture retention strategies as long as the surface drying potential of the slab is sufficiently low. This is generally not possible in the summer (i.e. external methods are required), but is often quite possible in the winter given the ambient conditions.

It also means the temperature can be TOO hot in the winter. If the boys on site turn up the salamanders such that the slab is as hot as it is in the summer, then you will have issues. The key is to maintain it close to the minimum required (10C), which can be done though it takes effort to monitor. Here’s a thread where I talk about it in somewhat more detail, especially the perils of trying to wet cure in the winter (short version: don’t do that).

As far as ill-effects to consider, the two big ones are compressive strength and stiffness. Since the modulus of elasticity is only loosely empirically correlated to compressive strength there is a chance that early stage drying completely messes up the assumed relationship (even if strength pans out); which can lead to excess deflection and floor finish problems. But again, the issue isn’t do you need to actively do something about moisture retention, it’s a question of whether or not the conditions are such that the drying potential of the slab surface is low enough that it won’t be a problem. A slab at 10C in winter conditions will not be a problem, and if you have site-cast cylinders showing appropriate compressive strength, you're golden.

If you are still concerned about it you should be able to speak with a manufacturer about a cure & seal product. While they can't generally be applied in winter conditions, they are mostly susceptible to substrate conditions rather than ambient conditions. And if the slabs are 10C, they should form a suitable substrate. Note that this comes with a pretty massive drawback that many floor finishes are not compatible with a cure and seal, so you'll have to blast track the slab prior to certain finishes (e.g. tiles, adhered vinyl, etc).

EDIT - Here are a few slides from an ACI course on shoring & re-shoring of high-rise structures that speak to the above. In the final slide the blue line is wet curing, and the green line is dry curing. It's clear then that the impact of curing, all else being equal, has an impact of the modulus of elasticity. However, what you really want to know is if your dry cured modulus in the winter conditions would be similar to what you could get with a water retention method in the summer. Unfortunately, there is no graph or chart or handbook for that determination. Only experience, really. And in my experience this is exactly what you would achieve assuming the workers on-site did not overcook the slab with the salamanders (or overload the early age slab - that is more critical in winter).



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[KootK]

And a special shoutout to KootK! You’re a legend in my office!

That made my day, thank you.

I'd like to hear your insights on flat slab curing in cold weather.

Not much to add other than you are right to be concerned. On a Canadian high rise that I was involved in, the hoarding blew open during a wind storm, the slab froze prior to curing, and we wound up having to chip the frozen top off and cast it back as a gross topping. We had the help of a materials engineer but, still, I'd have to imagine there's a cost in there some place from a performance perspective.

 

[Sibroe]

From all the replies, clearly the heating requirements are necessary and not optional in cold-weather. However, moisture retention is not as clear-cut.

The low-temperature heating approach proposed by Enable might work as a form of moisture retention. This could arguably count as “other moisture-retaining methods” as specified by Clause 7.7.2.1 of A23.1-14. However:

• I have yet to find a single technical document supporting this method explicitly.
• On-site crews clearly don’t have the knowledge, or the motivation, to fine-tune the heating for moisture retention.
• Even with good heating below, nothing is done to protect the top surface from large temperature differentials.

Also, looking at high-rise project specs from other firms, all I see is a generic requirement to comply with A23.1, nothing specific about moisture retention in cold-weather concreting.

My takeaway :

• Yes, this method is common practice in high-rise construction
• However, as a consulting engineer, I don’t have the technical basis to approve it, because it directly contradicts several clauses in A23.1 and ACI standards. Accepting this method as “industry standard” would be like saying multiple clauses in A23.1 and ACI standards are useless and not mandatory.
• The cure & seal approach is interesting, if it’s compatible with the flooring system.

If anyone else has more experience with this, I’d like to hear your thoughts.

Cheers!

 

[Enable]

I generally agree with your synopsis but have a few caveats to add.

There is technical literature on rates of evaporation and its impact on concrete curing, just generally not in the cold concrete literature (e.g. you wont find it in ACI 306 or in CIP 27 by NRMCA). I've typically found things of this nature in the Hot Concreting literature (ironically), and usually as they relate to plastic shrinkage rather than drying shrinkage. See here for a good article in Tilt-Up Today on that front.

If you go with the cure & seal route, it is even more important to ensure there are no foul-ups with the heating because it forms a film that traps moisture that'll go ahead and explode (leading to blisters) if the heat is turned off prematurely.

For cold weather concreting you'll most commonly find that insulated curing blankets are recommended. These act as both a thermal cover and as a vapor barrier. That said, there use in mitigating drying is somewhat limited since they do not adhere to the surface and most vapor is transported via air movement rather than diffusion (see Building Science article). Adhered poly is better for this purpose. Also, even if insulated blankets are used, you'll find that the next day they are kicked all over the place since the workers have to move on top of the slab (and they are dangerous to walk on). So for 95% of suspended slabs above grade out there that are placed in the winter, they get no topside protection after 12 - 24 hours (even if specified). If it was a major problem, we would know about it. Of course it very well can be a problem (i've been involved in forensics, and as mentioned in the other thread, I've seen these things happen) but if it was en masse, we would hear about it far more often.

My recommendation would be to place it on the contractor as a means and methods and to verify using in-situ cores. Field cured cylinders will give you a lower-bound, but since they should be located outside of the cure box, they may be more detrimentally affected by the ambient conditions than the slab itself (that has thermal mass). This is part of the reason that ACI 306 does not recommend their use as verification (though, as I said, we use them as a lower bound). Note: you still need cylinders in the cure box down below to be picked up for labs tests the next day.

Here are a few good presentations on the topic and other resources. Cant add any more than what you've already discovered. Best of luck!

Presentation 1
Presentation 2
CIP 27
ACI 306